Particularly in the case of analogue multiplier circuits which are designed and operated as radiofrequency mixers, a signal with a fixed, previously known DC component has to be provided at the signal input at which the useful signal is fed in. Depending on whether the input of the radiofrequency mixer is designed as a current input or as a voltage input, the DC component of the input signal has to be provided either as a DC voltage offset or as a DC current offset.
In mobile radio applications, two functional units are usually provided, namely a baseband signal processing and also a radiofrequency processing usually designed using analogue circuitry, which are normally arranged in separate integrated circuits (ICs). In the transmission direction, signals to be transmitted are processed in a baseband processor in accordance with the desired channel coding and modulation methods and converted into analogue, that is to say time- and value-continuous, signals at the output of the baseband block in a digital/analogue converter. This analogue signal provided at the output of the baseband chip is usually provided as a complex-valued signal with an in-phase component I and a quadrature component Q, I and Q signal component usually being carried as symmetrical or differential signals.
In the subsequent functional block, which effects the frequency conversion onto a radiofrequency carrier by means of a modulation, for example a quadrature modulation, provision is made of the radiofrequency mixers described in the introduction, which enable the frequency conversion from the baseband to a frequency in the radiofrequency range and, on the output side, are coupled to a transmission antenna, for example. The baseband processing block described and also the functional block of analogue design connected downstream are usually arranged on a respective integrated semiconductor chip. Since a signal with a fixed, previously known DC component has to be provided on the input side at the radiofrequency mixer, it is necessary for the connected baseband chip to provide a matching DC component at its output.
In order to ensure operation at a desired operating point of the frequency mixer, the DC voltage or DC current component on the input side is permitted to vary only within defined tolerances. However, the different baseband chips available have different DC components at the output which, moreover, are additionally subject to manufacturing tolerances.
It is an object of the present invention to specify a circuit arrangement for regulating a signal DC component which can be connected to an input of a radiofrequency mixer and enables the connection to different baseband signal processing circuits, and also a mobile radio transmitter having the circuit arrangement.